Add cdf and logcdf for Multinomial distribution

[rainerrodrigues]

This PR implements the cdf and logcdf functions for the Multinomial distribution, filling a gap in the multivariate function and addressing user requests.

Closes #1779

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.86%. Comparing base (a7ed7b6) to head (520cb09).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/multivariate/multinomial.jl	0.00%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2059      +/-   ##
==========================================
- Coverage   86.65%   85.86%   -0.80%     
==========================================
  Files         148      145       -3     
  Lines        8919     8723     -196     
==========================================
- Hits         7729     7490     -239     
- Misses       1190     1233      +43     

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